Three-dimensional force and torque converter

ABSTRACT

A three-dimensional force and torque converter unit for measuring an external force or torque applied to the unit and converting it into a signal, whereby the signal may be used to control a system or device incorporating the converter unit. The converter unit includes a controller formed with four spaced apart arms having six or more degrees of constraint. A force or torque may be applied to the tip portions of each of the arms via a gripping means. Sensors are used to measure the deflection of the arms under an applied loading or torque and an output signal is generated.

FIELD OF THE INVENTION

The present invention relates to control devices and more particularlyis concerned with such devices which respond to input forces or torquesin three dimensions and permit a control signal to be derived forcontrolling a machine such as a computer controlled system or the like.However, devices embodying the invention may be applied to other uses.

BACKGROUND OF THE INVENTION

The present inventor is also an inventor of inventions in this fieldrelating to three dimensional force and torque sensing devices which arethe subject of U.S. Pat. Nos. 4,811,608; 5,222,400; 5,706,027 and5,798,748.

A further prior published proposal in the field is U.S. Pat. No.4,589,810 Heindl et al.

In recognising this other prior published material, the inventor doesnot admit that any of these other proposals are necessarily known topersons working in the field or of that of common general knowledge inany particular country.

The inventors prior U.S. Pat. No. 4,811,608 discloses a six arm devicewhere the arms are orthogonally arranged and responses in the arms toforce or torque with respect to any axis in three dimensions aremonitored using sensors.

The inventor has now appreciated that new and useful alternatives to hisown prior art and other prior art items disclosed above would be highlyadvantageous and the present invention is concerned with suchalternatives.

SUMMARY OF THE INVENTION

In summary the invention may be described as a controller having fourand only four arms extending from a body portion which is adapted tosupport the device, the arms being spaced from one another in threedimensions and the device having six or more degrees of constraint, tipportions of each of the arms engaging in connection means providingrestricted relative motion, the connection means being attached to agripping means which can apply force and/or torque in a threedimensional sense, the device including response detection means formonitoring responses in at least three of the four arms to provide anoutput signal representative of force and/or torque applied through thegripping means.

In some embodiments the device is arranged to control a system with thesignal.

The arms may be arranged in a tetrahedron shaped envelope and optionallyare almost equally spaced from one another in a symmetrical sense withincluded angles of approximately 109°. However a small degree ofnon-symmetry is advantageous to ensure there is some preloadingmechanically which addresses friction issues yet provides a device inwhich the computer based system can rapidly perform the relevantcalculations that derive an accurate output signal.

Most usefully the arms are constrained such that the device has eightdegrees of constraint.

This may be achieved by the tip of each arm having a ball element whichis slidable along a cylindrical bore associated with the connectionmeans and rotatable within reasonable limits inside the bore. Thus eachsuch connection has freedom to engage in translational movement alongthe axis of the bore and limited freedom to rotate. The ball joint isthus constrained in two directions defining a plane at right angles tothe axis of the bore and there are four dimensions of freedom in totaland two constraints at each joint.

Optionally, the sensors for monitoring response in the arms are disposedaround a circular path in a plane. The sensors may advantageously be anoptically based system.

The optical system can detect very accurately extremely smalldeflections in the arms responsive to the applied force or torque.

Another embodiment is one in which six sensors are provided in an arrayso that displacements in an X-Y set of directions for each of the fourarms is achieved giving eight readings which can be resolved to give therequired output signal.

Another advantageous embodiment of the present invention includes aplurality of optical sensors as component parts of the responsedetection means. These optical sensors are concentric and disposed onthe same plane.

The six sensors may optionally be configured in pairs around three ofthe four arms.

The present invention, embodiments of which have been described above,may be usefully arranged as a component of a computer system wetherincorporated as an external facility or as an integral sub-system.

By way of technical background, an explanation of principles which mayfurther explain the invention or some of its embodiments will be given,but the applicant is not to be bound by the completeness or correctnessof this explanation. Further features of a preferred embodiments willalso be explained.

The constraint relationship between two bodies can be determined bysumming the constraints of the joint or joints between the two bodiesexcluding mechanisms which have special geometric alignments. Aperfectly constrained device would have exactly six degrees ofconstraint. Perfectly constrained designs require high joint tolerancesto avoid a rattling due to the joint clearances or to avoid excessivefriction of the joints due to interference. In practice a slightinterference renders the product unusable so perfectly constraineddesigns tend to exhibit a small amount of rattle due to the clearancesin the joints. It is also desirable to provide a small amount of dampingthrough some friction of the joints.

When a control device having a displaceable grip is designed, it isuseful to recognise that when the grip is released damping avoidsvibration issues and avoids the requirements of a very lightweight grip,as is the case with purely spring-based designs. The friction of aperfectly constrained design, when the grip is released, is onlydependent upon the weight of the grip and the frictional properties ofthe materials and hence is not adjustable in a typical design.

Overconstrained designs can be easily preloaded by slightly offsettingeither side of a joint. Optionally only a small overconstraint is usedto avoid tolerancing issues. A preferred embodiment of the presentinvention is slightly overconstrained with eight degrees of constraint.This allows the arms of the tube protrusions to be offset slightlyrelative to the connection means such as the cylindrical bores tointroduce a slight preload when the device is at rest.

Durability of a design is impacted heavily by the wear characteristicsof a joint. In perfectly constrained designs with point contact a smallamount of wear increases the slop of the joint resulting in increasedrattle of the device. The present preferred embodiments have linecontact joints that wear much more slowly than point contact. Inconjunction with a small preload the device does not exhibit slop.

The preferred embodiment has a central body and arms moulded as a singleunit to form a four-armed, generally star-shaped body which forconvenience in this specification will be known as a “tetra-star” toprovide rigid mounting of the arms of the body and to reduce cost. Acomplex tool is required to mould the central star part and each arm isformed by three sections of the tool. The preferred embodiment hasspherical tips that engage with bores in an outer ball or shell whichforms the grip. The mould has three parting lines. To avoid any flashfrom affecting the operation of the ball-in-hole joints, the idealspherical surface is optionally cut back along the parting lines with acylindrical surface so the flash will not touch the surface of thecylindrical bore associated with the outer ball.

In the preferred embodiment, there is an inner ball structure formounting the tetra-star and comprising a lower and an upper section.Four holes in the inner ball are provided for the cylindrically boredextensions from the outer ball to pass through and engage thetetra-star's arms. These holes also limit the range of motion of theextensions and prevent the arms from being overstressed. Impact loadsare passed directly from the extensions to the inner ball structurethereby avoiding damage of the tetra-star's arms so that a robust designis achieved.

Preferred embodiments use infrared LEDs and photodiodes to detect thetetra-star's arm displacements. Only six sets of sensors are requiredfor the full 3D force and 3D torque computation. These are optionallyarranged as three pairs with one arm having no sensors. Two pairs on twoarms and the other two arms with a single sensor is also possible butless desirable. Similarly eight sets of sensors could be used with apair for each arm. Each arm would optionally have the optical axesperpendicular to each other.

In the preferred embodiment a shadow mask technology is used for sensingthe displacement using an infrared LED and an infrared photodiode. Theuse of infrared provides greater immunity from ambient light affectingthe measurement. Light falling on the photodiode from the LED generatesa small current. As the arm deflects, the amount of light varies and inturn the amount of current varies. Greater linearity is achieved bykeeping the voltage across the photodiode constant using an appropriatecircuit. Each LED/photodiode pair has a characteristic loss factormeasured as the ratio of the LED drive current vs. the photodiode outputcurrent with no shadow. This is typically around 200:1. For goodaccuracy the drive circuitry and/or computation needs to compensate forthe variation in loss factor.

The preferred embodiment has ball-in-hole joints being 2degree-of-constraint joints. These have line contact between thespherical ball-tip surface and the whole surface.

DESCRIPTION OF THE FIGURES

For exemplification only the invention will be described with referenceto the following illustrative drawings:

FIG. 1 is a schematic three dimensional representation of a base unit ofa three dimensional control device, eg: for controlling computers;

FIG. 2 is a schematic vertical cross section through the device andhaving a generally spherical gripping cap for manual manipulation tooperate the device;

FIG. 3 is a schematic three dimensional view of a tetra-star componentused in the device;

FIG. 4 is a schematic three dimensional representation from the interiorof one of the segments of the cap of the device and used for grippingpurposes; and

FIG. 5 is a three dimensional exploded view of the device of FIGS. 1-4in the form of a practical embodiment.

FIG. 6 is a schematic view similar to the view of FIG. 3, however thetetra-star component is viewed along the axis of one of the arms. Thisview also shows one of the optics sub-assemblies. The tip of the arm hasbeen excluded to provide a better view of the optics sub-assemblies.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The principal components of the device comprise a tetra-star body 10base, an inner bowl shaped cap 12 and an outer cap 13 formed fromsegments, one of which is shown in FIG. 4.

The tetra-star 10 has four arms 14 extending along respective axes fromcentral body 15, the axes, being substantially uniformly geometricallydisposed relative to one another. Each arm 14 has an elongated reducedcross-section cylindrical portion 14A extending from a tapered base 16and leading to a tip 17 having an enlarged head with, the surfaceprofile including substantially a spherical portion 18 with a flattenedend face 19. A series of structural webs 20 are individually formed onthe tetra-star body.

The inner cap 12 has apertures 30 for accommodating tubular retainers 24associated with the outer cap 13 and thereby limited displacement of thecap 13 (which acts as a grip).

As most clearly seen in FIG. 2 one of the arms extends substantiallyvertically upwards and, as described above, a preferred embodiment hasoptical sensing for detecting flexing in the arms. FIG. 2 showsschematically a photo detector unit 21 having a light omitting diode(LED) 22 and photo detector 23. Each of the arms 14 is constrained withline contact in a respective tubular retainer 24 which is integrallyformed with and projects inwardly from the respective cap segments of 13to engage the tips 17.

Referring now to FIGS. 5 and 6 small cut-outs 31 in each of the threelower printed circuit boards (PCB) 32 provides clearance for theassembly of the photo detector unit 21 into the inner cap 12. Each ofthe three lower PCBs 32 mounts a printed circuit board interface 34.There are three triplets of optics subassembly supports 35 protrudingfrom the structural webs 20 to easily and accurately mount respectiveoptics subassemblies 36 which include the PCB 32 and two photo detectorunits 21.

The PCB interface 34 provides interconnections for the opticssubassemblies 36 and mounts interface electronics (not shown). A ribboncable (not shown) is soldered to the PCB interface 34 and runs inside astem 11 for connection to external electronics (not shown).

The top and bottom sections of the inner cap 12 respectively includethree pairs of clips 37 and three pairs of clip apertures 38 forinter-engagement. The stem 11 has three screw bosses (not shown) formounting the device to a base (not shown), a ribbon cable exit slot anda keying slot to ensure the device is mounted correctly. The edges ofthe segments of the outer cap 13 have interlocking tabs 39 for mutualattachment and assembly. These tabs 39 require all four parts of theouter cap 13 to be assembled at the same time. The interlock design ofthe tabs 39 require a simple two-part moulding tool for manufacture.Although the segments of the outer cap 13 mechanically engage, thesegments are glued for strength.

FIG. 6 clearly shows how the arms 14 are offset from the light omittingdiodes 22 and corresponding photos detectors 23 such that the variationin light due to the deflection of the arms 14 can be easily measured.

The tetra-star 10 is designed for plastic injection moulding. Notably,the spherical portion 18 of each arm 14 needs to be accurate and hassections profiled to keep any moulding flash below the spherical portion18. The type of plastic needs to have a good fatigue life to handle therepetitive bending stresses imposed on the arms 14 and it should havelow friction with the outer cap material. Delrin® is a suitable materialfor the tetra-star 10.

The arrangement is such that the application of force or torque throughthe outer cap 13 with respect to any axes is detected by acharacterising flexing in the arms. This flexing can be detected andcomputation determines the appropriate signal to be directed to a devicesuch as a computer.

As the outer cap 13 is moved, the four tubular retainers 24 push on thefour arms 14 deflecting them so they oppose the displacement of theouter cap 13. Ignoring the very small and hence insignificant frictionalcomponents, each arm tip 17 force vector can be considered as a 2D forcevector lying in a plane normal to the corresponding axis of the tubularretainer 24. A simplifying assumption is made that each plane remainsstationary as the outer cap 13 moves. The very small errors due to thisassumption are insignificant. The deflection of each arm tip 17 isproportional and in the same direction as the 2D force vector. Usingstandard engineering mathematics, each 2D force vector acting through aarm tip 17 can be transformed into a 3D force vector and a 3D torquevector acting through the centre of the device. The 3D force vector and3D torque vector acting on the outer cap 13 is then calculated bysumming the four 3D force vectors and summing the four 3D torque vectorsrespectively.

The force vector 13 acting on an arm tip 17 is proportional to thedeflection measured by the photo detector unit 21 (or sensor 21) locatedpart way down the length of the arm 14. The ratio of the force on thearm tip 17 to the measured deflection is constant and can be measuredexperimentally or calculated from an arm's spring constant combined withgeometric calculations of the shape of a deflected arm 14. Given theconstant ratio, the force is easily calculated from the deflection bymultiplication.

From engineering theory a minimum of six single value sensors arerequired to measure a simultaneous 3D force vector and 3D torque vector.Clearly, a device with four pairs of sensors, a pair for each arm, isfunctional. A device with three pairs of sensors can be used if thefourth 2D force vector can be calculated from the other three. Considerthe device of FIG. 5 where the lower three arms 14 have sensors 21 butthe top arm 14 does not. Using each of the three measured 2D forcevectors the force component tangential to a circle, centred on thecentre of the device and passing through the centre of the top arm tip17, is calculated. These three force vector components are thenmathematically rotated so as to act through the centre of the top arm'stip 17. These three force vectors are then summed to calculate the 2Dforce vector associated with the fourth arm 14.

It is helpful to consider the simple situation where the outer cap 13 ispushed downwards by a force acting through the centre of the device. Thetop arm 14 does not deflect but the lower three arms 14 deflectdownwards sharing the load equally. The required tangential componentshappen to be the same as their respective 2D force vectors. Rotatingthese force vectors so that they act through the centre of the top arm14 results in three equal force vectors acting 120° to each other andtherefore adding to zero as expected.

It is also theoretically possible to have a device with two pairs ofsensors 21 on two arms 14 and two single sensors 21, appropriatelyoriented, on the other two arms 14.

In this specification, the word “comprising” and its variations, such as“comprises”, has a meaning such that the word does not precludeadditional or unrecited elements, substances or method steps, inaddition to those specifically recited. Thus, the described apparatus,substance or method may have other elements, substances or steps invarious embodiments of the invention. The purpose of the claims is todefine the features which make up the invention and not necessarily allfeatures which a working embodiment of the apparatus, substance ormethod, to which the invention defines, may have. The apparatus,substance or method defined in the claims may therefore include otherelements, steps or substances as well as the inventive elements, stepsor substances which make up the invention and which are specificallyrecited in the claims.

1. A controller comprising a device having a body portion and four andonly four arms extending from the body portion which is adapted tosupport the controller, the arms being spaced from one another in threedimensions and the device having six or more degrees of constraint, tipportions of each of the arms engaging in connection means providingrestricted relative motion, the connection means being attached to agripping means which can receive and transmit an applied force and/ortorque in a three dimensional sense, the controller including responsedetection means for monitoring responses in at least three of the fourarms to provide an output signal representative of a three-dimensionalforce and/or a three-dimensional torque applied to the gripping means.2. A controller as defined in claim 1, wherein the response detectionmeans has means for directly monitoring response in three and only threeof the four arms and the device further comprises means for calculatingfrom data representing the monitored response in the three arms thevalues of a response in the fourth arm.
 3. A controller as defined inclaim 1, and further comprising a system connected to receive the outputsignal and to be controlled.
 4. A controller as defined in claim 1,wherein the arms are arranged in a tetrahedron shaped envelope and arealmost equally mutually spaced in a symmetrical sense with a smalldegree of non-symmetry to provide pre-loading at the connection means.5. A controller as defined in claim 1, wherein the device has structureto provide eight degrees of constraint to the arms.
 6. A controller asdefined in claim 1, wherein the tip of each arm has a portion with asubstantially part-spherical profile and is slidable along a cylindricalbore associated with the connection means and the arm is rotatablerelative to the axis of the bore.
 7. A controller as defined in claim 1,wherein each of the response detection means includes a plurality ofoptical sensors each of which has an emitter and a detector, the opticalsensors being arranged substantially in the same plane and havingrespective optical axes transverse to the axis of the associated arm. 8.A controller as defined in claim 7, incorporating a total of 6 opticalsensors disposed in pairs around three of the four arms.
 9. A controlleras defined in claim 1, and including a total of eight sensors providedin an array so that displacements in an X-Y set of responses for each ofthe four arms is achieved giving eight readings which can be resolved togive the required output signal.
 10. A computer system comprising acontroller including four and only four arms extending from a bodyportion which is adapted to support the device, the arms being spacedfrom one another in three dimensions and the device having six or moredegrees of constraint, tip portions of each of the arms engaging inconnection means providing restricted relative motion, the connectionmeans being attached to a gripping means which can receive and transmitan applied force and/or torque in a three dimensional sense, the deviceincluding response detection means for monitoring responses in at leastthree of the four arms to provide an output signal representative of athree-dimensional force and/or a three-dimensional torque to thegripping means, the output signal being arranged to control the computersystem.
 11. A computer system as defined in claim 10, wherein theresponse detection means has means for directly monitoring response inthree and only three of the four arms and the device further comprisesmeans for calculating from data representing the monitored response inthe three arms the values of a response in the fourth arm.
 12. Acomputer system as defined in claim 10, wherein the arms are arranged ina tetrahedron shaped envelope and are almost equally mutually spaced ina symmetrical sense with a small degree of non-symmetry to providepre-loading at the connection means.
 13. A computer system as defined inclaim 10, wherein the device has structure to provide eight degrees ofconstraint to the arms.
 14. A computer system as defined in claim 13,wherein the tip of each arm has a portion with a substantiallypart-spherical profile and is slidable along a cylindrical boreassociated with the connection means and the arm is rotatable relativeto the axis of the bore.
 15. A computer system as defined in claim 10,wherein each of the response detection means includes a plurality ofoptical sensors which are concentric and disposed on the same plane. 16.A computer system as defined in claim 15, incorporating a total of 6optical sensors disposed in pairs around three of the four arms.
 17. Acontroller comprising four and only four operative arms extending from abody portion which is adapted to support the controller, the arms beingspaced from one another in three dimensions and the device having six ormore degrees of constraint, tip portions of each of the arms engagingconnectors providing restricted relative motion and being attached to ahand grip which can receive and transmit an applied force and/or torquein a three dimensional sense, the controller including responsedetectors for monitoring directly responses in at least three of thefour arms to provide an output signal representative of athree-dimensional force and/or a three-dimensional, torque applied tothe hand grip and the device further comprising means for establishingthe response in each of the four arms for computing the output signal,the controller further comprising an output signal connector forconnection to a system to be controlled, and wherein the arms arearranged within a tetrahedron shaped envelope and are almost equallymutually spaced in a symmetrical sense with a small degree ofnon-symmetry to provide pre-loading at the connection means, and thearms are constrained such that the device has eight degrees ofconstraint.
 18. A computer system having software for control by aspatial controller, and further comprising a spatial controllercomprising four and only four arms extending from a body portion whichis adapted to support the controller, the arms being spaced from oneanother in three dimensions and the device having six or more degrees ofconstraint, tip portions of each of the arms engaging in connectionmeans providing restricted relative motion, the connection means beingattached to a gripping means which can receive and transmit an appliedforce and/or torque in a three dimensional sense, the controllerincluding response detection means for monitoring responses in three ofthe four arms to provide an output signal representative of athree-dimensional force and/or a three-dimensional torque applied to thegripping means and the device further comprising means for calculatingfrom the monitored response in the three arms the values of a responsein the fourth arm, and wherein the controller is arranged to control thecomputer a system with the output signal, and wherein the arms arearranged in a tetrahedron shaped envelope and are almost equallymutually spaced in a symmetrical sense with a small degree ofnon-symmetry to provide pre-loading at the connection means, wherein thearms are constrained such that the device has eight degrees ofconstraint.